That's not the point. Jawbone (and similar devices such as FitBits and Nike FuelBands) can detect whether the user is sleeping or not; this post asserts that the earthquake is the primary cause of the spike in sleep disruption at 3AM, which is a valid and supported hypothesis.

As far as I know, these devices cannot detect the cause of death.

But the device detects movement doesn't it? Is it not able to detect seismic activity or any other force? So if a person was in a car accident or in an earthquake that would be something that can be measured.

There are now devices that can tell what exercise you are doing just by being on your wrist.

I don't know much about this device but I guess it doesn't do continuous synchronization. I believe the user need to initiate it which won't happen if he die.